Approximately 280 million tires are discarded each year in the United States, only 30 million of which are retreaded or reused, leaving roughly 250 million scrap tires to be managed annually. Besides the need to manage these scraps tires, it has been estimated that there may be as many as 2 to 3 billion tires that have accumulated over the years and are contained in numerous stockpiles. The continuously rising prices of natural rubber provide an economic driving force to the environmental motivation to recycling scrap tires.
A typical scrapped automobile tire weighs 9.1 kg. Roughly 5.4-5.9 kg (13 lb) consists of recoverable rubber, composed of 35 percent natural rubber and 65 percent synthetic rubber. A typical truck tire weighs 18.2 kg and also contains from 60 to 70 percent recoverable rubber. Truck tires typically contain 65 percent natural rubber and 35 percent synthetic rubber. The majority of modern tires are steel-belted radials, containing 10-15% metals and 10% cords (e.g. polyester, nylon or rayon).
Approximately 45 percent of the 250 million tires generated annually are disposed of in landfills, stockpiles, or illegal dumps. About 7 percent are exported to foreign countries, 8 percent are recycled into new products, and roughly 40 percent are used as tire-derived fuel, either in whole or chipped form. Currently, the largest single use for scrap tires is as a fuel in various industries. At least 9 million scrap tires are processed into ground rubber annually. Ground tire rubber is used in rubber products (such as floor mats, carpet padding, and vehicle mud guards), plastic products and as a fine aggregate addition (dry process) in asphalt friction courses. Crumb rubber has been used as an asphalt binder modifier (wet process) in hot mix asphalt pavements.
The tire rubber waste is divided into categories defined by their size and method of production, i.e. slit tires, shredded tires or chipped tires, ground rubber and crumb rubber.
Rubber recycling process begins with shredding. After most of the steel and reinforcing cords are removed, a secondary grinding takes place, and the resulting rubber powder is ready for product remanufacture. The manufacturing applications that can utilize this inert material are restricted to those which do not require its devulcanization. In the rubber recycling process, devulcanization begins with cleavage of the sulfur-sulfur bonds which cross-linked the vulcanized rubber molecules, thereby facilitating the formation of new cross-linkages. Two main rubber recycling processes have been developed: the modified oil process and the water-oil process. With each of these processes, oil and a reclaiming agent are added to the reclaimed rubber powder, which is subjected to high temperature and pressure for a long period (5-12 hours) in special equipment and also requires extensive mechanical post-processing. The reclaimed rubber from these processes has altered properties and is unsuitable for use in many products, including tires. Typically, these various devulcanization processes have failed to result in significant devulcanization, have failed to achieve consistent quality, or have been prohibitively expensive.
Currently, tire reinforcing fiber (or tire cords) has very few uses in recycling and poses another significant problem in tire recycling industries. A rare example for a method for recycling tire cords is provided by U.S. Pat. No. 3,468,974 which teaches a molding composition containing 64-91% tire cord (polyamide) and 3-36% vulcanized rubber which is produced from extrusion pelletizing tire cord material. On the other hand production of products made of rubber originated from tires requires a step of devulcanization of the rubber. For example, in WO 2009/019684 there is disclosed a method for manufacturing a polymeric plastic product from used mineral oils, waste of hydrolysis of vegetable oils and/or animal fats as well as scrap automobile and/or other tires and/or other rubber waste.
Mixing Plastic with vulcanized rubber is known to recycle vulcanized rubber. For example, U.S. Pat. Application No. 2001/0056155 provides compression moldings from a mixture of ultra-low density polyethylene and a filer which may comprise recycled rubber.
U.S. Pat. Application No. 2005/0279965 describes a method for producing a composite material comprising mixing crumb rubber from recycled tires, plastic and asphalt in a high shear mixer.
U.S. Pat. Nos. 6,558,773 and 6,703,440 disclose a compression molding product made by blending and heating together rubber and a binder (e.g. ultra low density polyethylene) and extruding the blend followed by compression molding into a desired product.
U.S. Pat. No. 6,169,128 teaches a method for processing discarded plastic and rubber with a binder to obtain a processable material and useful products therefrom.
WO 2004/074594 teaches a panel for roofing or siding applications preferably made of a blend of rubber tire and drums.